1. Field of the Invention
This invention generally relates to the digital encoding of the luminance and color components of a color video signal and, in particular, to an adaptive technique which bases color component encoding on characteristics of the luminance component.
2. Description of the Prior Art
Even though color video signals are often represented as luminance and color components, which are encoded separately, it is well known that the interrelationship between these components can be used to advantage in increasing overall encoding efficiency. For example, in U.S. Pat. No. 3,803,348 issued to J. O. Limb and C. B Rubinstein on Apr. 9, 1974, it was recognized that significant changes in chrominance information generally accompany significant changes in the luminance signal or, stated differently, that luminance edges usually have accompanying color transitions. Accordingly, Limb et al arranged for transmission of average chrominance values until a significant luminance change occurred, whereupon an updated chrominance value was transmitted. While this technique was successful, it was nevertheless realized in U.S. Pat. No. 3,860,953 issued to C. C. Cutler, J. O. Limb and C. B. Rubinstein on Jan. 14, 1975, that important chrominance changes sometimes occur alone. To account for this, the luminance signal was intentionally distorted to signal to the remote receiver that an updated chrominance value was arriving. In another arrangement, it was shown by A. N. Netravali and C. B. Rubinstein in U.S. Pat. No. 4,023,199, issued May 10, 1977, that busy picture areas, as indicated by large luminance signal activity, could tolerate chrominance encoding errors better than flat picture regions. Accordingly, the quantizer characteristics of the chrominance encoders were adapted to the luminance signal activity in order to conserve bandwidth.
While the above systems are successful in utilizing the interrelationship of the luminance and color components of a color video signal so as to reduce redundancy in the overall encoded version thereof, it still appears that the correlation between certain characteristics of these components has not been fully exploited. Accordingly, the broad object of the present invention is to increase the efficiency and accuracy of color video signal encoding by employing luminance component characteristics to predict those of the color component. Other objects are to permit increased encoding efficiency with apparatus which is not unduly complex or costly. A still further object is the provision of apparatus that may easily be added to presently available equipment to attain the desired result.
As used herein, increased encoding efficiency can mean either increased encoding accuracy or picture quality with a constant transmission bit rate, or a reduction in the bit rate required to represent a video signal with a given picture quality.